1. Field of the Invention
This invention relates to a method of producing a silicon single crystal to be used as a semiconductor material.
2. Description of Related Art
Generally, Czochralski (CZ method) has been widely employed to obtain silicon single crystals. A growing apparatus used in the above CZ method is shown in a schematic sectional view of FIG. 1. Referring to the drawing, a crucible 11 set inside a chamber consists of an inner container 11a made of quartz in the shape of a bottomed cylinder and an outer graphite container 11b fitted outside the inner container 11a. A resistance-heating type heater 12 is disposed concentrically outside the crucible 11. A molten liquid 13 having a material of crystals melted by the heater 12 is filled in the crucible 11, in which is soaked a seed crystal 15 suspended by a lift axis 14 of a lift rod or a wire or the like. When the seed crystal 15 is pulled up while being rotated, the molten liquid 13 is coagulated at the lower end of the seed crystal 15, thereby to grow a single crystal 16.
In executing the above-described CZ method to grow a semiconductor single crystal, in many cases, impurity elements are added to the molten liquid 13 before the seed crystal is raised so as to adjust the resistivity and the type of conduction of the resultant single crystal 16. However, the added impurities are disadvantageously segregated in the growing direction of the single crystal 16, and therefore, the obtained single crystal 16 does not show uniform electric characteristics in the growing direction. The segregation results from the fact that the ratio of the concentration of impurities Cs in the single crystal 16 to the concentration of impurities C.sub.L in the molten liquid 13, Cs/C.sub.L at the growing interface between the single crystal 16 and the molten liquid 13, that is, the effective segregation coefficient Ke is not equal to 1. Supposing that Ke&lt;1, for example, the concentration of impurities C.sub.L in the molten liquid 13 is increased in accordance with growing of the single crystal 16, thus leading to the segregation of the single crystal 16.
A Double Layered CZ method has been known as a method of restricting the segregation depicted hereinabove. FIG. 2 is a schematic longitudinal sectional view showing a growing apparatus of crystals used in the Double Layered CZ method. The Double Layered CZ method arranges the coexistence of a solid layer 18 of a crystal material and a molten liquid layer 17 of the crystal material respectively in the lower part and the upper part of the crucible 11 under the control by the heater 12. While the concentration of impurities in the molten liquid layer 17 is kept constant, the seed crystal 15 is dipped into the molten liquid layer 17 and pulled up to grow a single crystal 16. In this case, there are two methods, i.e., a method with constant thickness and a method with varied thickness proposed to maintain the concentration of impurities in the molten liquid layer 17.
According to the method with constant thickness, the solid layer 18 is melted as the single crystal 16 is pulled up, thus keeping the thickness of the molten liquid layer 17 constant, while the impurities are continuously added thereby to hold the concentration of impurities in the molten liquid layer 17 constant. Japanese Patent Publication Nos. 34-8242 (1959), 62-880 (1987), Japanese Utility Model Application Laid-Open No. 62-150862 (1986), and Japanese Patent Application Laid-Open No. 63-252989 (1988), etc., reveal the method. On the other hand, according to the method with varied thickness, the crucible 11 or the heater 12 is moved up and down as the single crystal 16 grows, so as to change the thickness of the molten liquid layer 17, so that the concentration of impurities in the molten liquid layer 17 is held constant without adding impurities when the single crystal 16 is being pulled up, as is disclosed in Japanese Patent Application Laid-Open Nos. 61-205691 (1986), 61-205692 (1986), 61-215285 (1986) and the like.
The single crystal 16 obtained by the above CZ method contains oxygen in addition to the impurity elements. As a part of the inner container 11a of quartz is dissolved into the molten liquid 13, the eluting oxygen is supplied to the single crystal 16. FIG. 3 is a schematic diagram of a mechanism for taking the oxygen from the molten liquid 13 to the single crystal 16. As shown in FIG. 3, the amount of oxygen supplied to the single crystal 16 is determined by the elution from the quartz crucible 11, the transportation b to the whole molten liquid 13 because of the convection and the evaporation c from the surface of the molten liquid 13.
As described hereinabove, the single crystal 16 produced by the CZ method includes oxygen of approximately 13.times.10.sup.17 -17.times.10.sup.17 atoms/cc. It is thus difficult to obtain a single crystal of the lower concentration of oxygen by the CZ method.
In the MCZ method whereby a magnetic field is impressed from outside of the crucible 11 used in the CZ method to control the convection, since the supplying amount of oxygen to the silicon single crystal is decreased owing to the control of the convection b, it becomes possible to produce silicon single crystals of the oxygen concentration not larger than 10.times.10.sup.17 atoms/cc. However, the apparatus for the MCZ method is complicated and the production cost is undesirably increased.